What do you mean by "survey"? Do you mean the results of a survey published somewhere? If so, I'd go with Report. If you mean the raw data of a survey, I'd similarly use Report (eventually Zotero will add a Dataset item type, which would be even better). Do you mean the test form/questions/manual for a survey? For that, I'd also use Report.

Lacking a proper field I cite those sources as books and leave fields open.

For the Zotero people: GESIS, the German data archiv that hosts the Eurobarometer surveys, demands the following citation if you use its data:
Commission of the European Communities (2012): Eurobarometer 6 (Nov 1976). GESIS Data Archive, Cologne. ZA0990 Data file Version 1.0.1, doi:10.4232/1.10859

They explain:
Please use this information for the bibliographic citation. It refers to the actual version of the data file. A citation for older versions may be found under 'Errata & Versions'.

This citation is composed of:
[Principal Investigators] ([Version Year]): [Title]. [Data Collector]. GESIS Data Archive, Cologne. [Study Number] Data file Version [Version Number], [DOI].

If there are many principal investigators a name for the group is used instead of the person names.
If person names are used, a maximum of 10 person names are stated, after that 'et.al.' is used for abbreviation.

The year refers to the data file and is taken from the date of the version, except
if the version date is empty or '2010-04-13'. In this case a special 'publication year' is used, if available. If not available 'n.d.' is stated, because not in all cases the original publication year is known.

In some cases the data collector is stated in addition.